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SubjectRe: [PATCH] ARM: mm: Regarding section when dealing with meminfo
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On Thu, 2011-01-20 at 18:45 +0900, KyongHo Cho wrote:
> Sparsemem allows that a bank of memory spans over several adjacent
> sections if the start address and the end address of the bank
> belong to different sections.
> When gathering statictics of physical memory in mem_init() and
> show_mem(), this possiblity was not considered.
>
> This patch guarantees that simple increasing the pointer to page
> descriptors does not exceed the boundary of a section
...
> diff --git a/arch/arm/mm/init.c b/arch/arm/mm/init.c
> index 57c4c5c..6ccecbe 100644
> --- a/arch/arm/mm/init.c
> +++ b/arch/arm/mm/init.c
> @@ -93,24 +93,38 @@ void show_mem(void)
>
> pfn1 = bank_pfn_start(bank);
> pfn2 = bank_pfn_end(bank);
> -
> +#ifndef CONFIG_SPARSEMEM
> page = pfn_to_page(pfn1);
> end = pfn_to_page(pfn2 - 1) + 1;
> -
> +#else
> + pfn2--;
> do {
> - total++;
> - if (PageReserved(page))
> - reserved++;
> - else if (PageSwapCache(page))
> - cached++;
> - else if (PageSlab(page))
> - slab++;
> - else if (!page_count(page))
> - free++;
> - else
> - shared += page_count(page) - 1;
> - page++;
> - } while (page < end);
> + page = pfn_to_page(pfn1);
> + if (pfn_to_section_nr(pfn1) < pfn_to_section_nr(pfn2)) {
> + pfn1 += PAGES_PER_SECTION;
> + pfn1 &= PAGE_SECTION_MASK;
> + } else {
> + pfn1 = pfn2;
> + }
> + end = pfn_to_page(pfn1) + 1;
> +#endif

This problem actually exists without sparsemem, too. Discontigmem (at
least) does it as well.

The x86 version of show_mem() actually manages to do this without any
#ifdefs, and works for a ton of configuration options. It uses
pfn_valid() to tell whether it can touch a given pfn.

Long-term, it might be a good idea to convert arm's show_mem() over to
use pgdat's like everything else. But, for now, you should just be able
to do something roughly like this:

- page = pfn_to_page(pfn1);
- end = pfn_to_page(pfn2 - 1) + 1;
-
- do {
+ for (pfn = pfn1; pfn < pfn2; pfn++) {
+ if (!pfn_valid(pfn))
+ continue;
+ page = pfn_to_page(pfn);
+
total++;
if (PageReserved(page))
reserved++;
else if (PageSwapCache(page))
cached++;
else if (PageSlab(page))
slab++;
else if (!page_count(page))
free++;
else
shared += page_count(page) - 1;
page++;
- } while (page < end);
+ }

That should work for sparsemem, or any other crazy memory models that we
come up with. pfn_to_page() is pretty quick, especially when doing it
in a tight loop like that.

-- Dave



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